But since anyone using an EVO in the US is on Sprint, and has unlimited data, it hardly seems to matter.
As for the battery, that's what car chargers are for.
Update: A Google rep wrote to us, stating that while Navigation requires a data connection to plot its routes, search, and make detours and re-routes. But, once you’ve been driving a while, “as long as you don’t stray from the route, you will continue to get voice guidance even without a data connection.”